There are many methods and apparatus to accelerate charged particles (such as electrons, protons, and ions) to high energies. One such apparatus is known as a two-beam accelerator (TBA).
There are three different types of TBA's. Although their operating parameter regimes and the principle of operations are very different, they share one common feature: they all employ a high current electron beam (known as the driver beam or primary beam) to accelerate a low current beam (known as the secondary beam or the accelerated beam) to high energies.
The first type of TBA was proposed by Voss and Weiland. In this TBA, the primary beam consists of rings of electrons. These electron rings propagate near the outer wall of the accelerating structure. They generate an electromagnetic wakefield in a transient manner. This wakefield first propagates radially outward, toward the outer wall where it is reflected. Upon reflection, the electric field polarity of the wakefield reverses. As this reflected wakefield propagates radially inward, its amplitude increases geometrically, and is used to accelerate an on-axis secondary beam. This method of acceleration relies on the transient excitation and is not tuned to the resonant cavity mode. When the acceleration scheme is extrapolated to the lower energy regime of practical importance, it does not provide phase focusing for either the primary or the secondary beam.
The second type of TBA employs a highly relativistic electron beam as a driver. The beam's energy is converted into the microwave region of the electromagnetic spectrum through a klystron mechanism. The microwaves thus generated are piped into a separate accelerating structure to drive the secondary beam. This accelerator is not compact because a high energy beam from a linear induction accelerator is used as the driver. Since the primary beam is already very energetic, to modulate such a beam requires multiple cavities, extending over a substantial distance. To generate sufficient radio frequency (rf) power, and then to transport this rf into the accelerating structure, requires a complicated rf waveguide structure. The beams occupy separate structures.
The third type of TBA uses a modulated intense relativistic electron beam (MIREB) and is disclosed in U.S. Pat. No. 4,780,647 to Friedman. The modulated beam is terminated at a single gap, at which the entire available power of the primary beam is converted into rf power, which is delivered to a separate accelerating structure that houses the secondary beam. This device makes use of the fact that intense relativistic electron (IREB) (&lt;1 MeV) are easily modulated by microwaves from a magnetron. However, the action on the intense beam by a single gap leads to violent and uncontrollable power conversion on the one hand, and the formation of virtual cathodes on the other. In addition, it does not provide phase focusing of the secondary beam. The primary beam and the secondary beam also occupy separate structures.
The U.S. Pat. No. to Friedman, 4,215,291, discloses a collective particle accelerator including an IREB generator. A secondary electron beam propagates in a direction opposite the driver electron beam.
The U.S. Pat. No. to Schoen, 4,570,103, discloses a particle beam accelerator which requires an intense laser.